On Thu, Jul 1, 2021 at 10:50 AM Peter Collingbourne [off-list ref] wrote:

On Thu, Jul 1, 2021 at 8:51 AM Catalin Marinas [off-list ref] wrote:

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Hi Peter,

On Wed, Jun 30, 2021 at 04:29:31PM -0700, Peter Collingbourne wrote:

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If a user program uses userfaultfd on ranges of heap memory, it may
end up passing a tagged pointer to the kernel in the range.start
field of the UFFDIO_REGISTER ioctl. This can happen when using an
MTE-capable allocator, or on Android if using the Tagged Pointers
feature for MTE readiness [1].

When we added the tagged addr ABI, we realised it's nearly impossible to
sort out all ioctls, so we added a note to the documentation that any
address other than pointer to user structures as arguments to ioctl()
should be untagged. Arguably, userfaultfd is not a random device but if
we place it in the same category as mmap/mremap/brk, those don't allow
tagged pointers either. And we do expect some apps to break when they
rely on malloc() to return untagged pointers.

Okay, so arguably another approach would be to make userfaultfd
consistent with mmap/mremap/brk and let the UFFDIO_REGISTER fail if
given a tagged address.

This approach also seems reasonable. The problem, as things stand
today, is that UFFDIO_REGISTER doesn't complain when a tagged pointer
is used to register a memory range. But eventually the returned fault
address in messages are untagged. If UFFDIO_REGISTER were to fail on
passing a tagged pointer, then the userspace can address the issue.

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When a fault subsequently occurs, the tag is stripped from the fault
address returned to the application in the fault.address field
of struct uffd_msg. However, from the application's perspective,
the tagged address *is* the memory address, so if the application
is unaware of memory tags, it may get confused by receiving an
address that is, from its point of view, outside of the bounds of the
allocation. We observed this behavior in the kselftest for userfaultfd
[2] but other applications could have the same problem.

Just curious, what's generating the tagged pointers in the kselftest? Is
it posix_memalign()?

Yes, on Android that call goes into our allocator which returns the
tagged pointer.

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Fix this by remembering which tag was used to originally register the
userfaultfd and passing that tag back in fault.address. In a future
enhancement, we may want to pass back the original fault address,
but like SA_EXPOSE_TAGBITS, this should be guarded by a flag.

I don't see exposing the tagged fault address vs making up a tag (from
the original request) that different. I find the former cleaner from an
ABI perspective, though it's a bit more intrusive to pass the tagged
address via handle_mm_fault().

My preference is to fix this in user-space entirely, by explicit
untagging of the malloc'ed pointer either before being passed to
userfaultfd or when handling the userfaultfd message. How common is it
for apps to register malloc'ed pointers with userfaultfd? I was hoping
that's more of an (anonymous) mmap() play.

I think it is very unlikely for someone to use malloc'ed pointers with
userfaultfd.

At least we haven't seen any apps do this so far, and the tagged
pointers feature has been in Android since last year's Android 11
release. So maybe we can say this is uncommon enough that we can just
let userspace handle this. So we would do:

1. Forbid tagged pointers in the ioctl as mentioned above.
2. Fix the kselftest (e.g. by untagging the pointer, or making it use
mmap). A fix would probably be needed here anyway because we noticed
that the test is later passing a tagged heap pointer to mremap (and
failing).

The plan looks good to me. Using mmap (instead of posix_memalign)
seems like a cleaner fix to the kselftest as compared to untagging the
pointer everywhere.

I'd be okay with this approach but I'd first like to hear from
Alistair and/or Lokesh since I think they favored the approach in my
patch.

Peter